The invention relates to frequency transducers having means for precisely calibrating the upper and lower output current range limits thereof and, more particularly, relates to a frequency meter having means for easily calibrating the full scale and zero indications of the meter.
Heretofore it has been generally well-known to utilize diode pump circuits to provide a frequency transducing function. One common application for such frequency transducing circuits is in the manufacture of electronic tachometers. For example, U.S. Pat. No. 3,314,007, which issued on Apr. 11, 1967, illustrates such a diode pump circuit with a zener diode regulated input and a meter connected to provide a visual indication of variations in the frequency of monitored signals. It will be recognized that in using such a frequency transducer it is desirable to provide means for precisely calibrating the output indicating meter at both the maximum and minimum ends of its indicating range. A major obstacle to achieving such calibration is the inherent interaction in a simple diode pump circuit between its input and its output signals. Due to this interaction, a calibrating adjustment made for either end of the indicating range of the meter will normally cause a variance in the calibration of the meter at the other end of its indicating range. The present invention provides a solution to this common problem.
Before proceeding with a further description of the objects and advantages of the present invention, it is believed desirable to note the teachings of another U.S. patent, namely, U.S. Pat. No. 2,762,974 which issued on Sept. 11, 1956. Reference is made to this earlier patent merely to provide the reader with a clear understanding of the type of circuits referred to herein by the descriptive phrase "diode pump circuit." Thus, explained in that patent, such a simplified diode pump circuit is shown in FIG. 1 thereof. Of course, those skilled in the art will certainly appreciate that when such a diode pump circuit is used in a frequency transducer application the output terminals of the pump circuit may be connected to a variety of loads, such as to a meter, as illustrated in the aforementioned U.S. Pat. No. 3,314,007, or to an amplifier for supplying input signals to alternate indicating or load means. In all such cases, however, it is desirable to provide some means for effectively calibrating both ends of the range of output currents of the diode pump circuit.
Accordingly, an object of the present invention is to provide a frequency transducer circuit including a diode pump circuit in combination with a low impedance input transistor and two variable impedance means for separately adjusting the maximum and minimum limits of the output current range of the circuit in a manner such that interaction between the calibrating adjustments is minimized.
Another object of the invention is to provide a frequency transducer circuit having a diode pump circuit in combination with a low impedance input circuit and a variable impedance biasing circuit that are, respectively, adjustable to calibrate the full scale and zero settings of a meter connected to the output terminals of the transducer.
Yet another object of the invention is to provide an economical to manufacture, and reliable in operation, frequency transducer circuit incorporating variable impedance means for adjusting the full scale and zero settings of an indicating meter while eliminating interaction between the calibrating impedances.
Additional objects and advantages of the invention will be apparent to those skilled in the art from the description of it presented herein, considered with the accompanying drawing.